Select the first instance of a record

It sounds like you're looking for DISTINCT:

SELECT DISTINCT patientID FROM myTable

you can get the same "effect" with GROUP BY:

SELECT patientID FROM myTable GROUP BY patientID

You could use a CTE with ROW_NUMBER function:

WITH CTE AS(
    SELECT myTable.*
    , RN = ROW_NUMBER()OVER(PARTITION BY patientID ORDER BY ID)
    FROM myTable 
)
SELECT * FROM CTE
WHERE RN = 1

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